Establish a legal structure similar to European systems. Clerics are employed to record all land grants or financial affairs. Charters are legally binding documents. If they are broken, punishment would be justified.
Threaten them with the authority of the Crown. If they turn against you, they and all their families will be killed.
Sounds more like bribery to me! Also, you don't have the funds to do this. Reconsider your choice, and remember that money can't always buy respect.
Which card will you play?
